xref: /openbsd/share/man/man9/mutex.9 (revision 67035d4b)

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.Dd $Mdocdate: November 4 2019 $
.Dt MUTEX 9
.Os
.Sh NAME
.Nm mutex ,
.Nm mtx_init ,
.Nm mtx_init_flags ,
.Nm mtx_enter ,
.Nm mtx_enter_try ,
.Nm mtx_leave ,
.Nm MUTEX_ASSERT_LOCKED ,
.Nm MUTEX_ASSERT_UNLOCKED ,
.Nm MUTEX_INITIALIZER ,
.Nm MUTEX_INITIALIZER_FLAGS
.Nd interface to CPU mutexes
.Sh SYNOPSIS
.In sys/mutex.h
.Ft void
.Fn mtx_init "struct mutex *mtxp" "int wantipl"
.Ft void
.Fn mtx_init_flags "struct mutex *mtxp" "int wantipl" "const char *name" \
"int flags"
.Ft void
.Fn mtx_enter "struct mutex *mtxp"
.Ft int
.Fn mtx_enter_try "struct mutex *mtxp"
.Ft void
.Fn mtx_leave "struct mutex *mtxp"
.Fn MUTEX_ASSERT_LOCKED "struct mutex *mtxp"
.Fn MUTEX_ASSERT_UNLOCKED "struct mutex *mtxp"
.Fn MUTEX_INITIALIZER "int wantipl"
.Fn MUTEX_INITIALIZER_FLAGS "int wantipl" "const char *name" "int flags"
.Sh DESCRIPTION
The
.Nm
set of functions provides a non-recursive, interrupt-aware spinning mechanism
to ensure mutual exclusion between different CPUs.
.Pp
The
.Fn mtx_init
function is used to initiate the mutex pointed to by
.Fa mtxp .
When acquired, the mutex will cause the processor interrupt level to be raised
to
.Fa wantipl
if necessary.
.Pp
The
.Fn mtx_init_flags
macro is similar to
.Fn mtx_init ,
but it additionally accepts parameters for
.Xr witness 4 .
The pointer
.Fa name
differentiates a lock type.
Two mutexes have the same lock type only if they have been created by the same
occurrence of
.Fn mtx_init_flags
with the same pointer
.Fa name .
The
.Fa flags
parameter is a bitwise OR of the following options:
.Bl -tag -width MTX_NOWITNESS -offset indent
.It Dv MTX_DUPOK
Prevents
.Xr witness 4
from logging when a CPU acquires more than one lock of this lock type.
.It Dv MTX_NOWITNESS
Instructs
.Xr witness 4
to ignore this lock.
.El
.Pp
The
.Fn mtx_enter
function acquires a mutex, spinning if necessary.
.Pp
The
.Fn mtx_enter_try
function attempts to acquire a mutex.
.Pp
The
.Fn mtx_leave
function releases a mutex.
In case the acquisition of the mutex caused the interrupt level to be changed,
it is then restored.
.Pp
The
.Fn MUTEX_ASSERT_LOCKED
and
.Fn MUTEX_ASSERT_UNLOCKED
macros may be used to assert that a mutex is held locked or unlocked by
the current CPU.
.Pp
A mutex declaration may be initialised with the
.Fn MUTEX_INITIALIZER
macro.
When acquired, the mutex will cause the processor interrupt level to be raised
to
.Fa wantipl
if necessary.
.Pp
The
.Fn MUTEX_INITIALIZER_FLAGS
macro is similar to
.Fn MUTEX_INITIALIZER ,
but it additionally accepts parameters for
.Xr witness 4 .
See the
.Fn mtx_init_flags
macro for details.
.Sh CONTEXT
.Fn mtx_init
and
.Fn mtx_init_flags
can be called during autoconf, from process context, or from interrupt
context.
.Pp
.Fn mtx_enter ,
.Fn mtx_enter_try ,
and
.Fn mtx_leave
can be called during autoconf, from process context, or from any
interrupt context at or below the interrupt level
.Fa mtxp
was initialised with.
.Sh RETURN VALUES
The
.Fn mtx_enter_try
function will return non-zero if it succeeds in acquiring the mutex
.Fa mtxp ,
otherwise it will return 0.
.Sh SEE ALSO
.Xr witness 4 ,
.Xr msleep 9 ,
.Xr rwlock 9 ,
.Xr spl 9
.Sh HISTORY
The
.Nm
functions first appeared in
.Ox 3.6 .
.Sh AUTHORS
The
.Nm
functions were written by
.An Artur Grabowski Aq Mt art@openbsd.org .
.Sh CAVEATS
As these are spinning locks, don't sleep while holding one.
.Pp
Multiple mutexes may be nested, but not interleaved.
This is okay:
.Bd -literal -offset indent
mtx_enter(foo);
mtx_enter(bar);
mtx_leave(bar);
mtx_leave(foo);
.Ed
.Pp
While this is
.Fa not :
.Bd -literal -offset indent
mtx_enter(foo);
mtx_enter(bar);
mtx_leave(foo);
mtx_leave(bar);
.Ed